There are numerous known systems for plastic injection molding. In conventional plastic injection molding systems, plastic pellets are melted in an injection molding machine and advanced by a screw ram into a mold cavity. The mold cavity is formed between two mold halves (a core member and a cavity member), typically through one or more sprue bushings, a manifold and/or a hot runner system. The two halves of the mold are clamped, typically under high pressure, and the plastic is injected into the mold cavity, again under significant pressure in most instances. The molten plastic material in the cavity is allowed to cool and harden in the cavity, typically by a cooling system which circulates a cooling fluid through one or more of the mold members. When the part has sufficiently hardened, the mold is opened and the part is removed, typically by one or more ejector pins.
Some of the known systems utilize a gas in the injection molding process and are commonly known as “gas-assisted injection molding” systems. In these systems, the gas is injected into the molten plastic material through the plastic injection nozzle itself, or through ;one or more pin mechanisms strategically positioned in the mold, sprue bushings, manifold or hot runner systems. It is also possible to inject the gas directly into the molten plastic in the barrel of the injection molding machine. The gas, which typically is an inert gas such as nitrogen, is injected under pressure and forms one or more hollow cavities or channels in the molded part. The benefits of gas-assisted injection molding processes are well-known, and include the cost savings through the use of less plastic material, producing parts which are lighter in weight, and producing parts which have better surface definitions and finishes.
Another plastic injection molding system which utilizes gas injects the gas into the mold cavity along one or more exterior surfaces of the molded part. The pressurized gas forces the plastic against the opposite surface or surfaces of the mold cavity and forms a part with superior surface characteristics on the appearance surfaces.
One particular gas-assisted injection molding system utilizes a connecting spill-over cavity coupled to the mold cavity. Such system is shown, for example, in U.S. Pat. No. 5,098,637. In this system, a portion of the plastic from the mold cavity is displaced into the spill-over cavity when the charge of pressurized gas is introduced. This process has particular use for door and grip handles.
Although many of these gas-assisted injection molding systems operate satisfactorily and have produced commercially acceptable plastic injection molded parts and components, there is a need for improved systems and processes, and particularly those which do not utilize spill-over cavities.